CAMBRIDGE, Mass. — Each winter, wide swaths of the Arctic Ocean freeze to form sheets of sea ice that spread over millions of square miles. This ice acts as a massive sun visor for the Earth, reflecting solar radiation and shielding the planet from excessive warming.

The Arctic ice cover reaches its peak each year in mid-March, before shrinking with warmer spring temperatures. But over the last three decades, this winter ice cap has shrunk: Its annual…